1. Field of the Invention
This invention relates to an improved process for thermally stabilizing photoresist images and is more particularly concerned with the thermal stabilization of photoresist images having high resolution geometries for use in microelectronic applications.
2. Description of the Prior Art
As technology in the semi-conductor industry advances there is an increasing need for photoresist systems which can provide high resolution images having lines which are often below 1 micron in width. Positive resist systems are gaining popularity for producing such images. Typical of such systems are those based on novolak resins, used in a solvent base with a photosensitizer such as an ester of 1-oxo-2-diazo-naphthoquinone-5-sulfonic acid. The photoresist system of this type is coated on an appropriate substrate such as a silicon wafer whose surface has been treated to form oxides, metals, nitrides, phosphides and the like. The coated substrate is covered with a mask, exposed through the mask using appropriate UV radiation (often monochromatic) and then developed using an alkaline developer. The image present on the mask is thereby reproduced on the substrate, the areas of the photoresist layer, which were exposed to radiation by passage through the transparent portions of the mask, having been rendered soluble in the alkaline developer and therefore having been removed during the developing step. The unexposed portions of the photoresist layer, corresponding to the opaque portions of the mask, remain on the substrate. The developed image layer and substrate is then exposed to a post-development bake, to cure the photoresist remaining on the substrate and enhance the adhesion thereof to the substrate, before subjecting the image and substrate to the pattern generation step. The thermal curing of the photoresist may also take place if the photoresist and substrate are subjected to a later step involving exposure to heat and, in such circumstances, a specific post-development bake may not be required. The latter comprises etching, ion implantation doping, metal deposition and the like to produce the final image (e.g. an integrated circuit) on the substrate. Thereafter, in a final step, the remaining photoresist is stripped from the substrate using appropriate solvents or other techniques known in the art.
A number of problems arise when applying such techniques to the production of high resolution images. The harsh environment, usually including elevated temperatures, created in the etching, ion implantation and like techniques employed in the pattern generation step of the process frequently causes the photoresist image to lose its integrity either by softening and flowing, edge rounding, charring, cracking and the like. This loss of integrity is reflected in loss of the desired features in the final product of the process. Further, in order to prevent attack on the photoresist leading to failure of the latter and thus attack on the underlying substrate in places which the photoresist was designed to protect, it is frequently the practice to employ thicknesses of photoresist layer which are high in relation to the line widths in the image. Ratios of photoresist thickness to line widths as high as 2:1 have been employed. This is commonly referred to in the art as the use of a high aspect ratio image. As will be apparent to one skilled in the art, the use of such high aspect ratios is wasteful of relatively expensive photoresist system and contributes significantly to the cost of the overall process.
It is accordingly desirable to be able to produce high resolution photoresist images which are also capable of surviving exposure to the high temperatures involved in the post-imaging processes without losing their integrity. A number of attempts to achieve such photoresist images have been reported. Illustratively, Ma U.S. Pat. No. 4,187,331 teaches the heat stabilization of a resist image layer by subjecting the latter to an electrodeless flow discharge under low pressure in an atmosphere containing an organic fluorine compound such as carbon tetrafluoride.
Verelst et al. U.S. Pat. No. 3,652,274 describes the preparation of a metal printing plate in which a photoresist image is produced on the metal substrate and the image is developed using a hydrophobizing agent in the development fluid in order to increase the resistance of the image to the etching fluid used in the subsequent step. The hydrophobizing agent can be a fluoroalkyl-substituted organic silane.
Tada et al. U.S. Pat. No. 4,454,222 teaches the preparation of high resolution photoresist images using as the photoresist resin a polymer derived from trifluoroethyl-2-chloroacrylate and employing certain ketones as developers for the exposed image.
Matthews U.S. Pat. No. 4,548,688 describes hardening the photoresist by exposure to UV radiation. This method can cause difficulty in subsequent stripping of the photoresist as well as introducing an additional equipment handling step in the overall process of producing the final product.
Chin et al. U.S. Pat. No. 4,125,650 describes hardening photoresist images by chemically bonding a layer of a quinone-diazide hardening agent to the image. Cratering of the unexposed photoresist in positive photoresist images is said to occur due to gas evolved by the diazo compound during the baking step of the process. The coated image is also rendered more difficult to remove from the substrate after pattern generation has been completed.
It has now been found that high resolution photoresist images which are stabilized against distortion and other forms of thermal degradation during post-imaging treatments of the attached substrate, can be produced by a novel process which will be described hereinafter.